Finally I caught Sam and got a SIM! Although nano SIMs are already 'sold out', you can cut it with scissors!
NOTE: You have to provide a UK address and your credit card info when registering giffgaff's account (even if your credit card address is not associated with a UK address.) In that case, just enter UK address (such as Thistle Barbican City). They don't check the credit card address.
--[[U:Takot]]